A metropolitan area network system and an optical access network system are a focus of current research. The metropolitan area network sends user services distributed in different places (a company, an organ of government, a smart residential district, a commercial-residential building, a hotel, a school, and the like) to a backbone layer after integrating, sorting, and converging the user services to a maximum extent, so that network hierarchy becomes very clear and efficiency is greatly improved. A plurality of access technologies are available for a broadband metropolitan area network when an optical fiber is used as a transmission medium. An Ethernet access technology and a passive optical network (PON) technology are currently mainstream technologies.
Generally, the metropolitan area network system and the optical access network system have such features as simple system and low costs. After emergence of the optical fiber with a loss of 20 dB in 1970s, people firstly developed and used the optical fiber in intensity modulation (IM)—direct detection (DD) mode for communication. This mode has such advantages as simplicity, cost-effectiveness, and ease of adjustment. In addition, there is another type of fiber optic communications: coherent optical communications. Compared with the IM-DD, the coherent optical communications can not only modulate amplitude of an optical wave, but also perform frequency shift keying or phase shift keying, such as binary phase shift keying, differential phase shift keying, and continuous phase frequency shift keying.
The coherent optical communications has a plurality of modulation modes, which facilitate a flexible engineering application, but increase system complexity and costs. Compared with the coherent optical communications, main advantages of the IM-DD communication mode are as follows: easy system implementation and low costs for components; it is applicable to the conventional metropolitan area network system and the optical access network system.
In a high-speed transmission system, a solution that combines the IM-DD with an orthogonal frequency division multiplexing (OFDM) technology is adopted. The OFDM technology is a special frequency division multiplexing technology. The OFDM is a type of multi-carrier modulation. According to its main idea, a channel is divided into multiple orthogonal subchannels; high-speed data signals are converted into multiple parallel low-speed subdata signals, and each low-speed subdata signal is modulated for transmission on a subchannel. In a frequency domain, these low-speed subdata signals are orthogonal to each other after being modulated to each subchannel, and the signals modulated to each subchannel are restored at a receive end by using a demultiplexing technology.
FIG. 1-a is a frequency domain diagram of the OFDM. In FIG. 1-a, each individual channel has seven subcarriers, and each subcarrier, represented by a different peak point, meets orthogonality in a whole symbol period. That is, a maximum power point of each subcarrier directly corresponds to a minimum power point of an adjacent subcarrier, so that the subcarriers can partially overlap without interfering with each other, thereby ensuring that the receive end can restore the signal without distortion. The OFDM technology, by overlapping the subcarriers, use a spectrum more efficiently. Although the solution that combines the IM-DD with the OFDM technology has such advantages as improved spectral efficiency and no need for a dispersion compensation fiber in a link, due to a limit on system costs, a distributed feedback laser (DFL), a direct modulation laser, or an electro-absorption modulated laser (EML) is often adopted in the system as an optical signal modulator; when these optical signal modulators perform an amplitude modulation, an accompanying phase modulation is inevitable, and an output optical signal has a phase shift as time changes. This phenomenon is called chirp.
After passing a standard signal-mode fiber, the optical signal with the chirp affects the signal at the receive end, causing a frequency deviation of the optical signal. Higher chirp causes a greater frequency deviation; optical signals with different frequencies have different frequency deviations. FIG. 1-b is a frequency domain diagram of the OFDM after the chirp is introduced at the receive end. It may be seen from FIG. 1-b that due to an introduction of the chirp, orthogonality of the optical signal at the receive end is destroyed, that is, the maximum power point of each subcarrier no longer directly corresponds to the minimum power point of the adjacent subcarrier, thereby causing a severe inter-symbol crosstalk. No related technical solution in the prior art can effectively resolve a problem of the frequency deviation of the optical signal caused by the introduction of the chirp.